Modem systems are widely used to communicate data over the public switched telephone network ("PSTN") and other telecommunication networks. A general block diagram of a typical modem system 100 is illustrated in FIG. 1. A first modem 102 may be operatively associated with any suitable data source, e.g., a first personal computer ("PC") 104. Similarly, a second modem 106 may be operatively associated with a second PC 108. Although not shown in FIG. 1, modem 102 or modem 106 may be integrated with PC 104 or PC 108, respectively.
When a data communication session is initiated, a data communication channel 110 is established between modems 102 and 106. Digital data from PC 104 may be processed by modem 102 in accordance with any number of well-known protocols, then transmitted over data communication channel 110 to modem 106. Upon receipt of the transmit signal, modem 106 processes the received signal to extract the original digital data, then presents the data to PC 108.
The broad functional overview set forth above applies to virtually all modem systems. However, the detailed operation of modem systems is often governed by national or international standards. For example, certain modem systems may be governed by ITU-T Recommendation VO.34, published September 1994 ("VO.34"), the entire content of which is hereby incorporated by reference. To ensure that individual modem devices are compatible with one another, VO.34 specifies: transmission data rates; encoding and decoding techniques; signals and sequences utilized during start-up, training, and other modes; line probing methodologies; electrical characteristics; and other operating parameters.
Once a communication session is established between the modem 102 and modem 106, the Phase 1 procedure set forth in VO.34 is performed. The Phase 1 procedure generally relates to the identification and selection of operating features supported by modems 102 and 106; this procedure is often referred to as the modem capabilities exchange procedure. For example, different modem devices may be configured to operate in accordance with any number of standardized (but not mandatory) functional protocols. Prior to entering the data mode, the modem devices exchange and/or negotiate their operating capabilities and select one or more protocols to be followed during the current communication session. Phase 1 is governed by ITU-T Recommendation VO.8, published 1994 ("VO.8"), and ITU-T Recommendation VO.8 bis, published August 1996 ("VO.8 bis"). The entire contents of VO.8 and VO.8 bis are hereby incorporated by reference. VO.8 bis dictates how modems 102 and 106 perform the identification and selection of operating protocols.
Following Phase 1, modems 102 and 106 enter Phase 2 to conduct line probing and ranging in accordance with VO.34. The receiver associated with modem 106 is trained for optimized performance at the designated data rate during Phase 3. Phase 4, which follows the Phase 3 training, is performed to enable modems 102 and 106 to exchange data rates and modulation capabilities, e.g., the enabling of shaping, non-linear mapping, and trellis code type. After Phase 4 is complete, the modem system may enter the data transmission mode.
Due to the various start-up routines and procedures, an undesirably long time may pass between the initialization of the communication session and the beginning of the data mode. This delay (i.e., start-up latency) is due in part to the large amount of data transmission and processing that is performed during capabilities exchanging, training, and other VO.34 start-up procedures. Furthermore, VO.8 and/or VO.8 bis is performed at a relatively slow data rate, e.g., 300 bps, rather than the relatively high data rates typically associated with the data mode. Accordingly, the exchange and processing of data during VO.8 and VO.8 bis has the side effect of delaying the data mode.
In prior art VO.34 modem systems, the Phase 3 training is performed in response to the primary data rate selected for use during the data mode. Training the modem system at the primary data rate utilizes a relatively large number of training parameters designed to assess the line conditions and tune the equalizers resident at the receive modem, e.g., modem 106. In addition, training for the primary data rate may also be more prone to errors; redundant transmission of training sequences may be required to reduce the likelihood of such errors. As a result, the rather long sequences transmitted during conventional Phase 3 training may exacerbate the problem of start-up latency in the modem system.